G1 interference with Cactus V4 triggers

I've just started using Cactus V4 triggers with my G1. First results show an interference pattern occupying the first quarter to a third of a landscape format frame.

This apparent interference remains regardless of changes to aperture and shutter speed - from 160th and slower. Interference disappears if I remove the transmitter from the camera hot shoe and connect it to the camera via short cable.

Anyone else experiencing similar problems. Any suggestions, apart from to stop being such a cheapskate!
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